756 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, 



to the diminution of the number of the branchiae in Cambarus and 

 Astacus, and to an important difference in the structure of those of 

 Engceus; and Hagen has pointed out some important peculiarities 

 of these organs in Cambarus ; while the remarkable fact, that the 

 appendages of the first somite of the abdomen are absent in many of 

 the Crayfishes of the southern hemisphere, has been duly noted by 

 Erichson and several other zoologists. 



Having recently had occasion to make a careful reexamination of 

 the structure of Astacus Jluviatilis, I found two minute filaments 

 attached to the epimera of the penultimate and antepenultimate 

 thoracic somites. The structure and the position of those filaments 

 led me to suspect that they must be rudimentary branchiae ' ; and as 

 the Australian Crayfishes appeared to me to be, on the whole, less 

 specialized forms than the European species, I thought that I should 

 probably find in them fully-formed functional branchiae occupying 

 the place of these rudiments. Through the kindness of my friend and 

 former pupil, Mr. J. Wood-Mason, a specimen of " Astacoides" 

 franklini was placed at my disposal ; and on examination, I not only 

 found the functional branchiae I sought, but discovered a number of 

 other interesting differences between the respiratory organs of this 

 Crayfish and those of Astacus. 



Following up the line of inquiry thus suggested, I have examined 

 examples of all the chief forms of Crayfishes at present known, with 

 the result of establishing some remarkable parallel relations between 

 the morphology and the distribution of these animals. 



In order to make these points clear, I must premise a fuller and 

 more precise description of the branchial apparatus of the common 

 Crayfish than has yet been given, in order that it may serve as a 

 standard of comparison for the branchiae of the other Crayfishes. 



II. The Modifications of the Branchiae in the Crayfishes. 



The Branchice of Astacus fluviatilis. 



When the branchiostegite of a Crayfish is removed, seven 

 branchiae are seen, running from the base towards the apex of the 

 branchial cavity, parallel with one another, and disposed in curved 

 lines, which are concave forwards and convex backwards. The length 

 of the branchiae gradually increases from the first to the sixth; the 

 seventh ascends as high as the sixth, but is rather shorter, in conse- 

 cpuence of the attachment of its base lying at a higher level. 



In each of the six anterior branchiae, a basal portion, a stem, an 

 expanded lamina, and an apical plume may be distinguished. The 

 basal portion (fig. 1, i, b) is broad, with a convex posterior and 

 inferior free edge, beset with long setae ; and it is articulated by its 



1 I have met with no allusion to these structures, unless the following pas- 

 sage in Brandt's and Ratzeburg'e description of the Crayfish (Med.-Zoologie, ii. 

 p. 61) refers to them : — " Ueber jeder der obersten Kiemen der beiden mittlem 

 Fusspaare steht etwa 1'" entfernt ein kleiner fiidenformiger, unten breiterer, 

 bartelahnlicher Tlieil." I do not quite see the applicability of " unten breiterer," 

 unless "unten" applies to the attached ends of the filaments; but in other 

 respe. ts the description fits the rudimentary branchiae very well. 



